The present invention relates to instruments for generating precision measurements of time intervals between measured events in pulse streams, and more particularly to characterizing noise in repetitive waveforms.
The semiconductor industry is continually striving to satisfy demands for higher data processing speeds, in connection with complex computers, high definition video graphics, telecommunications, and other fields that place a high premium on rapid transfer of large amounts of data. Microprocessor systems move data at speeds based on high frequency clocks that determine the rate at which signals are clocked through semiconductor circuitry. During this decade, typical clock frequencies have increased from about 33 MHz to about 200 MHz, and semiconductor devices currently under development are expected to operate under clock frequencies that exceed 1 billion Hz.
Typical microprocessors can have from 256 to 512 connector pins or pads for the input and output of data. Clocked signals exit the device through as many as 100 of the connector pins, and it is imperative to know the relationship of each clocked pulse to the pulses on the other output pins. Irregular timing, if not corrected, can cause errors in the semiconductor device.
When timing signals at higher frequencies, it becomes increasingly important to analyze, and if possible reduce, the intrinsic noise components of data and clock signals. This noise is known by several terms including jitter, wander, unintended modulation, and phase noise. Jitter, as such noise is referred to herein, concerns the instability of pulse streams, especially repetitive waveforms. Ideally, repetitive pulse streams are absolutely stable, in that each individual pulse or cycle has the same width or duration. Jitter represents a deviation, perhaps in picoseconds, from the ideal. As clock frequencies within semiconductor devices and other high speed applications increase, the jitter component becomes more pronounced. In high definition video graphic chips, jitter can cause a flicker or jumping of the video image. Jitter can cause glitches in audio devices, and disparity between output and input serial data in network applications. Nowadays, many semiconductors are designed to allow no more than a 500 picosecond (i.e. 500 trillionth of a second) error between one output pin and another. Tolerances in high definition video applications are more stringent, e.g. as low as 100 picoseconds. Accordingly, measurements of jitter and other aspects of timing are critical during the prototyping and development stages of semiconductor devices.
Therefore, it is an object of the present invention to provide an apparatus and method for more accurately characterizing the noise component of data and clock signals.
Another object is to provide a process for measuring and characterizing jitter in a manner that reduces jitter contributed by the measurement system.
A further object is to provide a system for sampling a waveform in a manner that improves the distinction between true noise frequencies and frequencies detected due to aliasing.
Yet another object is to provide a time interval measurement system in which a limited variance of a nominal or average sampling frequency, and an accumulation of multi-cycle durations to form sets of data corresponding to different cycle spans, lead to more rapid and more accurate measurements and characterization of jitter components in repetitive wave forms.